STUDY ON THE IMPACT OF HEAVY FLOODS ON ENVIRONMENTAL CHARACTERISTICS OF VEMBANAD BACKWATER

Chapter 3

3.2.6 Periyar

Periyar River is the longest river in Kerala state having a length of about 244 km
and a catchment area of about 5398 km2(Fig. 3.3). It originates from Sivagiri Hills of
Western Ghats. The average annual stream flow of Periyar was 4867 mm3. The river flows
along almost virgin forests in places such as Kokaripara, Neriamangalam, Edamalayar and
Malayattoor. At Aluva, the river bifurcates into two, Marthandavarma and Mangalapuzha
branches. The Mangalapuzha branch joins Chalakudy River and empties into the Arabian
Sea at Munambam while the Marthandavarma branch flows southwards, through the
Udyogmandal area and joins the Cochin backwater system at Varapuzha [Periyar Valley
Irrigation Project (PVIP), 1972]. Periyar River plays a major role in the economy of Kerala.
Major portion of Kerala's electrical power get through the hydroelectric projects in Periyar
River. Kochi city depend up on Periyar River for the drinking water source. Around 25 %
of Kerala's industries are situated along the banks of Periyar River.

3.2.7 Chalakudy

Chalakudy River or Chalakudy Puzha is the fourth longest river in Kerala. The river
flows through Palakkad, Thrissur and Ernakulam districts. It is formed by the confluence of
the Parambikulam, Kuriarkutty, Sholayar, Karappara and Anakkayam streams. Of these,
the Parambikulam and Sholayar originate from the Coimbatore district in Tamil Nadu and
remaining streams from hills of Palakkad district. The Parambikulam wildlife sanctuary is
drained by the streams of Chalakudy River. The river originate at an altitude of >1250 m
above msl. The river has a length of 130 km and it drains an area of 1404 km2 in Kerala and
300 km2 in Tamil Nadu. It joins the Periyar River at Elenthikara (near Puthanvelikkara,
adjacent to Manjali, North Paravur in Ernakulam district).Though Chalakudy River in strict
geological sense is a tributary of the Periyar River, for all practical purposes it is treated as
a separate river by Government and other agencies. According to the annual report of
National Bureau of Fish Genetic Resources Lucknow, the Chalakudy River is the richest
river in fish diversity perhaps in India. In addition to evergreen and semi-evergreen species,
the riparian forests of the region have been shown to be distinguished by the occurrence of
typical riparian species of plants. Of the 319 species of flowering plants identified from the
region, 24 are endemic species of the Western Ghats and 10 are rare and endangered.
Furthermore, the Chalakudy River is recognised for its rich diversity, as it contains 85
species of fresh water fishes out of the 152 species known from Kerala.Of these, 35 are
endemic species of the Western Ghats and nine are considered to be endangered. The
famous waterfalls, Athirappilly Falls and Vazhachal Falls are located on this river. The

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 71

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Thumboormoozhy Dam is constructed across this river for irrigation purposes. Besides, the
Parambikulam Dam has been built on the Parambikulam River, one of its five tributaries.

3.2.8 Karuvannur
The Karuvannur River or Karuvannur Puzha has its origins at Pumalai Hills in

Chimmony Wildlife sanctuary of Thrissur district. The Karuvannur River is formed by the
confluence of two rivers, Manali River and Kurumali River. The Manali tributary of
Karuvannur originates from Vaniampara Hills (> 365 m) and the other tributaries such as
Chimmony and Mupli from Pumalai at an elevation of > 1100 m above msl. The Peechi
part of Peechi-Vazhani wildlife sanctuary is drained by Manali tributary and Chimmony
wildlife sanctuary is drained by Chimmony tributary. The river flows west and splits in
two, one falling in Enamakkal Lake in Thrissur district and the other one into Periyar River.
The total length of the river is 40 km, drains an area of 1054 km2and gives drinking water
to many Panchayats in Thrissur district.

3.2.9 Keecheri
The Keecheri River or Keecheri Puzha is one of the small rivers in the state and is

practically dry during summer. It is a west-flowing river which has its origins at Machad
Hills in Thrissur district at an elevation of >365 m above msl. The river is 51 km in length
and empties to Arabian Sea at Chettuva Lake. It is linked with backwaters at Enamaakkal.
Choondal Thodu is the only important tributary of this river. It has a drainage area 401 km
and the river basin is located in the Thrissur district. The Vazhani part of Peechi-Vazhani
wildlife sanctuary is drained by the Keecheri River.

3.2.10 Puzhakkal
Puzhakkal is a westward flowing river in Thrissur district of Kerala. It originates

from Killannoor Hills at an elevation of > 150 m above msl and empties into Thrissur-Kol
wetlands. The total length of the river is 29 km and a total drainage area of 234 km2.
Parathodu, Nadathodu, Poomalathodu and Kattachirathodu are the maintributaries of this
river.

72 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 3.1 Riverine map of Vembanad wetland system
(Source: Kerala Soil Map by National Bureau of Soil Survey and Land Use Planning (ICAR),

Nagpur, India)

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 73

Chapter 3

Fig. 3.2 Catchment area of rivers draining into Vembanad wetland system

Fig. 3.3 Catchment area of Periyar River draining into Vembanad wetland system
74 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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3.3 Major reservoirs and dams in Kerala

There are 81 dams in Kerala, in which 59 dams are owned by the Kerala State
Electricity Board and which form around 45 reservoirs, 20 dams are owned by the Kerala
State Irrigation Department which form 20 reservoirs and the Kerala Water Authority vests
the control of 2 dams with 2 reservoirs. Three dams have no drainage area across the river
(i.e. Munnar Headworks dam, Lower Periyar dam and Maniyar dam). Besides this, 10 large
barriers are also present in the state. Of the 81 dams, 37 reservoirs are used for
hydroelectric power, 27 reservoirs are used for irrigation and 9 reservoirs are used for both
hydroelectric power and irrigation. Around 74 % of the total live water storage of Kerala is
accommodated in the 7 major reservoirs, each having a live storage capacity of more than
0.20 BCM (Table 3.1).

Table 3.1 Reservoirs having a live storage capacity of more than 0.20 BCM

Sl. No. Name of reservoir Live storage capacity (MCM)

1. Idukki 1460

2. Idamalayar 1018

3. Kallada 488

4. Kakki 447

5. Parambikulam (for use of TN) 380

6. Mullaperiyar (for use of TN) 271

7. Malampuzha 227

3.3.1 Periyar River basin (PRB)

The Periyar (244 km) is the longest river and PRB is the second largest river basin
of Kerala, with a catchment area of 5398 km2out of which nearly 98 % lies in the Kerala
state and drains parts of Idukki and Ernakulam districts of the state. The state wise
distribution of the drainage area is given in Table 3.2. The Periyar River system is mainly
regulated by 17 dams and reservoirs and 2 barrages, which are constructed for the purpose
of hydroelectric power generation as well as irrigation. Around 80 % of the hydroelectric
projects of the state are located in the Periyar River basin (Abe & Joseph, 2015).

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Table 3.2 State wise area distribution of Periyar sub-basin

Name of State Drainage area (km2) Percentage of total
drainage area

Tamil Nadu 114 2

Kerala 5284 98

Total 5398 100

Periyar River has a drainage area of 4,033 km2 upto CWC gauging station at
Neeleshwaram. Idukki, Idamalyar and Mullaperiyar are the three reservoirs with substantial
live storage capacity in Periyar sub-basin. The Periyar River has a catchment area of about
637 km2 at Mullaperiyar dam. The free catchment between Mullaperiyar and Idukki dam is
about 605 km2. Catchment area obstructed by Idamalayar dam is about 472 km2. Periyar
sub-basin comprised of about 50 % of the total live storage of the state (ie., about 2.92
BCM). The total storage of Idukki reservoir is about 1997 MCM at FRL of 732.43 m. It has
a total storage of about 537 MCM at MDDL (Minimum Drawdown Level) of 694.94 m.
The live storage between FRL and MDDL is about 1460 MCM. The Idamalayar dam is
located on the Idamalayar River, a tributary of the Periyar River. Its live storage is about
1018 MCM.

3.3.2 Pamba River basin

In Pamba River, two hydrological observation stations are maintained by CWC i.e.
at Kalloppara on river Manimala and Malakkara on river Pamba. The Pamba River
bifurcates at Pandanad, with one branch taking a western course. The Manimala joins the
Pamba in its Neeretupuram branch. After that the river flows north and falls into Vembanad
wetland through many branches, with the Pallathuruthy Aar and the Nedumudy Aar being
the important ones. There are 8 dams and one barrage in Pamba sub-basin. The total live
storage capacity is 487 MCM, which is 10.5 % of the average annual runoff of 4.64 BCM
(4640 MCM). Out of the total live storage capacity, only Kakki (447 MCM live storage)
has a significant storage and it is the major reservoir project in Pamba basin. Kakki
reservoir is built across the river Kakki, a tributary of Pamba River. It has a gross storage of
about 450 MCM at FRL of 981.46 m and storage of 7.6 MCM at MDDL of 908.3 m. In
Pamba sub-basin, the Kakki storage is about 92 % of the total live storage. Next to Kakki

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storage is the Pamba storage (live storage only 31 MCM). The total live storage of all other
reservoirs and barrages is only 9 MCM.

Fig. 3.4 Rivers and reservoirs influencing the Vembanad wetland system
3.4 Rainfall data

Kerala economy solely depends on monsoon rainfall for its water needs and the
rainfall in the state is mainly controlled by the south-west and north-east monsoons. But
recently a wide spread change can be seen in rain spell due to the climate change and the
change in monsoon wind pattern over the region. Kerala has an average annual
precipitation of about 3000 mm. However, according to Indian meteorological Department
data (IMD), the state witnessed 2346.6 mm rainfall as against 1649.5 mm (42 % above the
normal) during 2018 flood. The rainfall departure in Idukki was the highest viz. 92 %. The
district wise rainfall occurred during 1st June 2018 to 22nd August 2018 is shown inFig. 3.5.
3.4.1 Rainfall pattern in Kerala during flood, 2018

According to CWC, the rainfall obtained during 15-17th, August 2018 flood period
was entirely significant, with more than 800 mm rainfall obtained at Peermade rain gauge
station followed by more than 700 mm at Idukki. This resulted in severe flooding in 13
districts of the state, but only one district (Kasaragod) was out of this tragic flood. Kerala
experienced above than normal rainfall during June, July and August. June faced 15 %
more, July 18 % more and 1st to 19th of August, it was 164 % more (758.6 mm against the
normal of 287.6 mm) (Table 3.3).

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 77

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Fig. 3.5 District wise rainfall obtained during 1stJune 2018 to 22nd August 2018
(Source: Central Water Commission report, Govt. of. India - September 2018)

Table 3.3 Month wise normal rainfall, actual rainfall and percentage departure from normal

Period Normal Rainfall Actual Rainfall Departure from
(mm) (mm) normal (%)

June, 2018 649.8 749.6 15

July, 2018 726.1 857.4 18

1-19, August, 2018 287.6 758.6 164

Total 1649.5 2346.6 42

3.4.2 Rainfall obtained during 15-17th, August 2018 flood period

During 15-17th August 2018, the intense rainfall and storm was spread over the
entire state with maximum rainfall at Peermade, a place between Periyar and Pamba sub-
basins. Due to the severe storm, the gates of 35 dams were opened to release the flood

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runoff. All 5 overflow gates of the Idukki dam were opened, for the first time in 26 years.

Heavy rains in hilly districts especially Wayanad and Idukki causes heavy landslides,

causalities and crop loss and left the hilly districts isolated. The Cochin International

Airport, India's fourth busiest in terms of international traffic, and the busiest in the state
suspended all operations on August 15th until August 29th following flooding of its runway.
The severity of the flood has been compared with the flood of 16-18th July 1924 centred at

Devikulam in Kerala is shown in Table 3.4. According to the data, the 2-day and 3-day
rainfall obtained in Pamba, Periyar and Bharathapuzha sub-basins, during 15-17th August
2018 was almost comparable to the Devikulam flood of 16-18th July 1924. For the entire
Kerala, during 15-17th August 2018, the rainfall obtained was 414 mm while the same
during 16-18th July 1924 was 443 mm.

Table 3.4 Comparison of rainfall obtained in different sub-basins and rest of the Kerala
during 15-17th, August 2018 flood with Devikulam flood of 16-18th, July 1924

15 Aug 15-16, Aug 15-17, Aug 16 Aug
2018 2018
Sl. Name Area 1-day 2018 2018 1-day
No (km2) (mm) (mm)
132 2-day 3-day 155
129 83
176 (mm) (mm) 217
198 248
1 Rest of the Kerala 26968 114 279 364 182
128 141
2 Kallada 1139 180 208 289 83

3 Pamba 1620 397 538

4 Periyar 4035 452 588

5 Bharathapuzha 5784 297 373

6 Chaliyar 1992 256 331

7 Valapattanam 1019 263 336

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3.4.3 Rainfall obtained in Periyar River basin during flood period, 2018
The rainfall obtained during extreme rainfall event of 15-17th August 2018 in

Mullaperiyar, Idukki and Idamalayar catchments and remaining part of the sub-basin along
with estimated runoff during the same period are given in Table 3.5.

Table 3.5 Rainfall and runoff in Periyar sub-basin up to CWC G&D site

Catchment Area Rainfall Rainfall Rainfall Runoff Runoff Runoff
depth 15 depth 15-16, depth 15-17, 15Aug 15-16, 15-17,
Aug 2018 2018 Aug 2018 Aug 2018
(1-day) Aug 2018 Aug 2018 (2-day) (3-day)
(1-day)
(2-day) (3-day)

(km2) (mm) (mm) (mm) (MCM) (MCM) (MCM)

Free Periyar 2362 203 459 589 374 845 1084

Between 605 240 523 682 123 269 351
Idukki and
Mullaperiyar

Mullaperiyar 637 196 415 536 106 225 290

Idamalayar 472 179 394 496 72 158 199

Total 4076 190 454 584 675 1498 1925

According to the CWC report, the maximum discharge at Neeleshwaram G&D site
was about 8800 m3s-1 on 16th August 2018. The cumulative runoff for 15-17th August 2018,
computed from the Neeleshwaram G&D records was about 1.93 BCM, while the estimated
runoff from IMD rainfall was about 1.925 BCM for a runoff coefficient of 0.78 for free
catchment and 0.85 for catchments tapped by dams.

During the extreme rainfall event of 15-17th August 2018, the total release during
three days from Idukki reservoir was about 345 MCM (spill) and 30 MCM (power house
going to Muvattupuzha River) against the inflow volume of 435 MCM. Hence, about 60
MCM of flood runoff was absorbed by Idukki reservoir during 15-17th August. On
15thAugust 2018, the average release from Idukki reservoir was about 1100 m3s-1 with peak

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release of 1500 m3s-1 against the average inflow of 1640 m3s-1. Idukki reservoir received an
average 533 m3s-1 discharge from Mullaperiyar on 15th August 2018 with a peak discharge
of 760 m3s-1. On 16th August 2018, the average release from Idukki reservoir was about
1400 m3s-1 with peak release of 1500 m3s-1 against the average inflow of about 2000 m3s-1.
Idukki reservoir received an average 650 m3s-1 discharge from Mullaperiyar on 16th August
2018 with a peak discharge of 760 m3s-1. On 17th August 2018, the average release from
Idukki reservoir was about 1460 m3s-1 with peak release of 1500 m3s-1 against the average
inflow of about 1440 m3s-1. Idukki reservoir received an average 390 m3s-1 discharge from
Mullaperiyar on 17th August 2018 with a peak discharge of 590 m3s-1.

3.4.4 Rainfall obtained in Pamba River basin during flood period, 2018

Generally, the Pamba basin experiences good rainfall, moderate temperature and
humid atmosphere. The south-west and north-east monsoon have great influence over the
climatic condition of the basin. Even though the coastal regions of the basin experience hot
with high humidity, the hilly region is generally cold. The average annual rainfall in Pamba
basin varies between 2276 mm to 4275 mm. The rainfall obtained in Kakki dam, Pamba
dam and remaining part of the sub-basin along with estimated runoff during 15-17thAugust
2018 is given in Table 3.6.

Table 3.6 Rainfall and runoff in Pamba River basin up to CWC G&D site

Area Rainfall Rainfall Rainfall Runoff Runoff Runoff
depth 15 depth depth 15Aug 15-16, 15-17,
Catchment 15-16, 15-17, 2018 Aug Aug
Aug Aug Aug 2018 2018
Manimala 2018 2018 2018
G&D to
confluence (1-day) (2-day) (3-day) (1-day) (2-day) (3-day)
Manimala
G&D site to (km2) (mm) (mm) (mm) (MCM) (MCM) (MCM)
confluence
Pamba dam 700 175 388 526 92 204 276

93 175 388 526 12 27 37
75 207 449 586 12 25 33

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Kakki dam 177 196 394 522 26 52 69

Catchment up

to Malakkara 1369 181 409 551 185 420 566

G&D site

Catchment

between

Malakkara 63 110 197 280 5 9 13

G&D site and

Manimala

confluence

Total 2477 179 397 537 297 663 894

According to this, the estimated areal rainfall of Pamba sub-basin was about 179
mm, 397 mm and 537 mm respectively for 1-day, 2-day and 3-day rainfall of 15-17th
August 2018. For Manimala River up to CWC G&D site, the runoff volume of 1-day, 2-
day and 3-day have been estimated as 92 MCM, 204 MCM, and 276 MCM respectively
assuming a runoff coefficient of 0.75 corresponding to three day observed runoff of 277
MCM at Kallooppara G&D site. The same runoff coefficient has also been adopted for
Pamba sub-basin with estimated 1-day, 2-day and 3-day runoff of 223 MCM, 497 MCM
and 668 MCM upto Malakkara G&D site. From the flood hydrograph of Malakkara G&D
site total runoff in 3 days is about 533 MCM. The difference in volume may be attributed to
retention of overtopped water over river banks in nearby areas.

3.4.5 Combined runoff of Pamba, Manimala, Meenachil and Achankovil Rivers
during flood

The estimated runoff for a runoff coefficient of 0.75 from Pamba, Manimala,
Achankovil and Meenachil River systems up to Vembanad wetland during 15-17th August

2018 is given in Table 3.7.

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Table 3.7 Rainfall and runoff in Pamba, Manimala, Achankovil and Meenachil river systems
in Vembanad wetland

Area Rainfall Rainfall Rainfall Runoff Runoff Runoff
depth depth depth 15Aug 15-16, 15-17,
Catchment 15 Aug 15-16, 15-17, 2018 Aug Aug
2018 Aug Aug 2018 2018
2018 2018

(1-day) (2-day) (3-day) (1-day) (2-day) (3-day)
(km2) (mm) (mm) (mm) (MCM) (MCM) (MCM)

Achankovil 1359 122 231 329 124 235 336

Pamba & 2656 173 382 517 346 762 1030
Manimala

Meenachil 820 146 327 437 90 201 268

Total 4835 441 940 1283 560 1198 1634

3.4.6 Hydro-climatic conditions of Kerala during August 2018

According to the study conducted by Mishra et al. (2018), the long-term (1901-
2018) average annual precipitation in Kerala was approximately 2400 mm with a standard
deviation of 400 mm. Therefore the 117 years (1901-2018) of Kerala‟s observed rainfall
record reveals that the two wettest years occurred in 1924 and 1961, with an annual rainfall
of around 3600 mm. However in Kerala, the rainfall in 2018 (until August) was lower than
the rainfall recorded in 1924 and 1961. They also calculated the return period of extreme
rainfall in Kerala during August 2018 using GEV (Generalized Extreme Value)
distribution. They find that, the 1-day maximum rainfall (120.2 mm) averaged over the
entire state in August 2018 (occurred on 15th August) had a return period of about 75 years.
Moreover, the 2-day maximum rainfall (235.5 mm) (occurred on 15-16th August 2018), had
a return period of about 200 years. The 3-day maximum rainfall (294.2 mm) (occurred on
15-17th August 2018), was more than a 100-year event considering the record of 117 years.
Hence, from this, it is clear that, the 2-day maximum rainfall in Kerala during August 2018
was the most detrimental to the return period of more than 200 years.

The monsoon season of 2018 was anomalously wet between May 1st and August
21st, as the majority of the state received more than 1500-2000 mm rainfall. In 2018,

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rainfall in most of Kerala was 42 % above normal average, while between May 1st and
August 21st 2018, the southern region recorded more than 200 % rainfall. During 8-17th
August 2018, heavy rainfall occurred in much of the state. Majority of the state received
more than 500 mm rainfall with an excess of 40-50% during this period (8-17thAugust,
2018). The study of daily rainfall from 3-20th August shows that there was significant
rainfall during 8-9 August 2018, which continued in Kerala until 18thAugust. Rainfall
occurred on August 15th and 16th was anomalously higher, as most part of Kerala received
more than 200 mm rainfall each day. Before heavy rain that caused enormous flooding and
loss of life in Kerala, the persistent rainfall before 15-16th August could have produced
saturated conditions.

In India, severe precipitation events have increased in frequency and intensity over
the last few decades (Roxy et al., 2017; Guhathakurta et al., 2011; Goswami et al., 2006).
According to Fowler et al. (2010), frequent and extreme precipitation events cause
flooding, which have become common in India (Mohapatra and Singh, 2003). According to
the Fifth Assessment Report of the Intergovernmental Panel for Climate Change (IPCC)
2014, south-west season floods can be understood as an obvious illustration of the effect of
global climate change with very heavy rainfall in a short period of time. In India,
Mukherjee et al. (2018) have recorded a rise in extreme precipitation from anthropogenic
warming. In their study, they find that, 1-5 day heavy rainfall at 5-500 year return period is
increases by 10-30 % under the anthropogenic warming. Moreover, when compared to
north India, heavy rainfall in southern India increases with a much faster (18 %/K) rate in
response to warming. However, the attribution of a single extreme event to climate change
is difficult, despite the consensus of the increase in extreme precipitation under the
warming climate. For instance, According to van Oldenborgh et al. (2016), climate change
did not cause the flooding and heavy rain event in Chennai, 2015.

3.5 Impact of rainfall, river discharge and reservoir operation during 2018 flood on
the Vembanad wetland

The 2018 flood event in Kerala demonstratesthat, reservoirs can play a crucial role
in the flood situation. The storage of reservoirs during the monsoon season plays a
significant role, providing water for irrigation and producing hydroelectric power. Hence,
before the monsoon departs, most of the reservoirs optimise their storage. In Kerala there
are 57 large dams, out of which 4 dams are operated by Government of Tamil Nadu. The
total live storage capacity under these dams is 5.806 BCM. This is equal to 7.4 % of annual
average runoff of all 44 rivers in Kerala, which is about 78 BCM (Water Resources of
Kerala, 1974). Out of these dams, only 7 reservoirs are having a live storage capacity of

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more than 0.20 BCM and they constitute 74 % of the total live storage in Kerala. Before the
flooding, the storage condition of these major reservoirs (i.e. Idukki, Idamalyar, Kallada,
Kakki, Parambikulam, Mullaperiyar and Malampuzha) was anomalously high. All these
seven major reservoirs had storage much higher than their long-term (2007-2017) mean.
For example, on 8th August, 2018, six (out of seven) reservoirs were at more than 90 % of
their full reservoir level (FRL). Parambikulam had reservoir storage 99.5 % of its FRL
while in Idukki, Idamalyar, Kakki, Kallada and Malampuzha reservoir storage on 8th
August 2018, was 92.5 %, 97.3 %, 90.5 %, and 97.8 % of their FRL. Mullaperiyar was the
only major reservoir that had reservoir storage of less than 80 % of its FRL. During May 1st
to August 2018, excess rainfall (40-50 %) occurred in Kerala, which led to above average
reservoir storage. Reservoirs such as Idukki, Kakki, and Periyar were already almost full,
witnessed extreme rainfall of more than 500 years return period. Excess rainfall was
received by Idukki, Kakki, and Periyar reservoirs were 279 %, 700 %, and 420 %
respectively from their long-term mean between May and August in 2018. Kerala recorded
above normal rainfall in the 2018 monsoon season, which significantly contributed to the
storage of reservoirs. According to Mishra et al. (2018), the Kerala flood, 2018 was caused
by multi-day extreme rainfall and partly due to high reservoir storage. There are several
other small and medium sized reservoirs in Kerala that could have had elevated storage
before the flood. Therefore, most of the major reservoirs were almost full before the heavy
rainfall occurred on 15-17th August 2018. Therefore, there was no capacity for reservoirs to
handle the additional flow generated in the upstream catchment areas by unprecedented
extreme rainfall. Reservoirs had to release a significant amount of water in a short period of
time due to heavy rainfall after August 8th. A red alert was flashed on Idukki reservoir on
9th August 2018, which later on opened all the gates to release water to lower the reservoir
level (Anon, 2018).

In Pamba River basin, the live storage of Kakki reservoir is about 447 MCM. The
reservoir level on 8th July 2018 was 965.05 m i.e. 16.41 m below FRL. In terms of storage
volume, the live storage was 226 MCM i.e. 51 % of total live storage capacity. Afterwards,
there was continuous rain during 9-27th, July. As the reservoir was only half-full prior to
this spell of rains, there were no spills and reservoir level rose to 979.04 m on 28 July 2018
with a live storage of about 403 MCM. The reservoir was now 91 % full. So, the Kakki
reservoir absorbed this heavy spell of rain fully. However, as a result, it got very close to
FRL in July itself with only 39 MCM extra flood cushion available below FRL. The
releases from Kakki reservoir could not be made to deplete water level in Kakki reservoir,
as at that time the below MSL areas in Kuttanad region were already experiencing heavy

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inundation. Moreover, the Thottappally spillway at Vembanad wetland, which receives
water from Pamba, Manimala, Achankovil and Meenachil rivers, out of which only Pamba
basin is having Kakki dam as a control structure. The other three are uncontrolled rivers.
The Thottappally spillway has a discharging capacity of around 630 m3 s-1. Therefore, the
water takes time to pass through the spillway and get accumulated in the low-lying areas
around Vembanad wetland. So, when the low lying areas in Kuttanad region are already
experiencing inundation, the discharge from Kakki reservoir, makes it a complicated
situation.

If Kuttanad region was not flooded prior to the second spell of extreme rains,
preferably, when this spell of rain once abated, the reservoir level could have been brought
down to some extent, to moderate any future extreme flood events that might affects the
reservoir in the month of August. Unfortunately, this happened 11 days later. On 9th August
2018, the reservoir level was 981.25 m and it was nearly with and no spills from the dam.
Now, any flood event could have been moderated between the space available between
FRL (981.46 m) and MWL (982.16 m). Only about 20 MCM dedicated flood space is
available between FRL and MWL. As per dam site rainfall record, the rainfall during the
second event that occurred during 9-20th August 2018 was 1724 mm with 590 mm rainfall
in just two days i.e. 15-16th, August 2018. The maximum inflow in the reservoir was 835
m3s-1 with a corresponding release of 938 m3s-1. As there was no space left in the reservoir,
it could not provide any flood attenuation during this second event and the space between
FRL and MWL was quickly exhausted. The total flood peak observed in Pamba sub-basin
was 2900 m3s-1. Even if there was just 500 m3s-1 release from Kakki, the downstream flood
peak would still have been about 2400 m3s-1.

As per the report of Planning Commission (July 2008), the water carrying capacity
of the Vembanad wetland system was reduced to an appalling 0.6 BCM from 2.4 BCM as a
result of land reclamation. The Pamba reservoir (31 MCM) and Kakki reservoir (447
MCM), in the Pamba sub basin can hardly regulate 10.5 % of the average annual flow in
the Pamba River. All other storages in Pamba River are very small ones having no
appreciable storage capacity. Also, the other three rivers such as Manimala, Meenachil and
Achankovil have no storages on them. As part of the Kuttanad development scheme,
Thottappally spillway was constructed in 1954 for mitigating flood situation in Kuttanad,
by diverting flood water of Pamba, Manimala, Achankovil and Meenachil directly to the
sea. The Thottappally spillway consists of a leading channel 1310 m long 365 m wide with
a bridge cum regulator across the spillway channel. The bridge cum regulator is 365 m
along with 40 vents, each having 7.6 m clear span. Though the original discharge capacity

86 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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of the spillway was about 1812 m3s-1, it is reported that at present the average maximum
discharge passing through the spillway is limited to 630 m3s-1, which is almost one - third
of the design capacity of the spillway.

During 15-17th August 2018 rainfall, the runoff generated from Pamba, Manimala,
Achankovil and Meenachil Rivers was about 1.63 BCM (1630 MCM) against the 0.6 BCM
(600 MCM) carrying capacity of Vembanad wetland. Furthermore, the discharging capacity
of Thottappally spillway (630 m3s-1) was other major constraint for the disposal of runoff.
Considering the wetland carrying capacity of about 600 MCM and discharging capacity of
630 m3s-1 of Thottappally spillway and around 1706 m3s-1 present discharging capacity of
Thanneermukkom barrage, it can be concluded that out of 1.63 BCM the runoff generated
during the 15-17th and subsequent days in August 2018 rainfall, only about 0.605 BCM
runoff was possible to drain out of the Vembanad wetland. The remaining runoff volume of
about 1 BCM created the rise of the water level in the wetland and adjacent areas. This
continuous rising of water may be one of the reasons of overall change in the river
hydrodynamics of Pamba, Manimala, Meenachil and Achankovil River systems resulting
higher water level for a particular discharge in these rivers. Considering the high rainfall
during 15-17th, August 2018, the absence of substantial storage reservoirs in the upstream
of the major rivers, reduction of depth and shrinkage of carrying capacity of Vembanad
wetland, reduction of the capacity of Thottappally spillway and the structural limitations of
Thanneermukkom barrage may have worsened the flooding in Kuttanad region and the
water flows to the low-lying areas in the upper reaches of the backwater. This may be the
reason for the heavy flooding experienced in the low-lying areas closer to the Vembanad
wetland in the Alappuzha, Kottayam, Ernakulam, Thrissur and Pathanamthitta districts.
Several places in Chengannur, Kuttanad and Ambalapuzha taluks have been isolated
following an alarming rise in the water level. In Kuttanad taluk, Kainakary, Pulinkunnu,
Chambakulam, Edathua and Ramankary were badly affected. More than 200 relief camps
and 483 gruel centres have been opened in different parts of the district for around 1.25
lakhs people.

3.6 Effect of flood on the Western Ghats ecology with special reference to Vembanad
wetland

The Western Ghats is known to be one of the world‟s eight hottest biodiversity hot
spots and an ecologically sensitive region. It stretch 1600 km from the mouth of Tapti
River near the Gujarat and Maharashtra borders to Kanyakumari, Tamil Nadu southernmost
tip of India, covering six states; Gujarat, Maharashtra, Goa, Karnataka, Kerala and Tamil

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 87

Chapter 3
Nadu (Fig. 3.6).With its high stature, lush tropical rain forests, the vegetation reached its
greatest diversity towards the southern tip of Kerala. As a real water tower of the Indian
Peninsula, the Western Ghats are the source of several east and west-flowing rivers. Also,
there are a variety of natural as well as several man-made wetlands in the Western Ghats
region, that are valuable from the perspective of aquatic species and migratory waterfowl.
Besides serving as corridors, the riparian vegetation along the various east and west-
flowing rivers and streams of the Ghats shelters high levels of plant and animal diversity.
Hence, the entire Western Ghats region needs to be considered as ecologically sensitive as
being „upper catchment areas‟ critical for the sustainability of the rivers of the Indian
Peninsula and also as a repository of wetlands.However, many reasons have contributed to
this fragile habitat being disrupted and this has necessitated the restoration of the Ghats and
the sustainable use of its resources.

Fig. 3.6 The Western Ghats boundary
(Source: WGEEP report, 2011)

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Today, Kerala has over 5,000 quarries, out of which over 2,000 are in the Western
Ghats. Out of 81 dams in Kerala, 35 are hydroelectric projects which contributed to the
destruction of over 350 km2 of evergreen forests and altered the riverine ecosystem in
many ways. Many dams commissioned before 1971, the reservoir capacity has been
significantly reduced due to silting and thus unable to hold water as per their designed
capacity. When united Kerala was created in 1957, 36 % Kerala‟s land area constituted
forests, according to 2016 economic survey only 3.9 % is „dense‟ forests. In an
ecologically fragile state where 75 % of the land has a gradient of above 20 %, the loss of
dense forest cover of this magnitude is an invitation to disaster. Massive forest losses in
the catchments of rivers and dams have contributed to excess runoff during the extreme
rains in August 2018 in Kerala, adding to severity of floods. Deforestation for planting
cash crops like tea, coffee, cardamom and pepperresults massive soil erosion leads
landslides in hilly districts of Kerala. That had happened in many parts of Kerala during
the monsoon season. Heavy landslides may change the geography of an entire land.
Wherever landslides happened, there were granite quarries on the other side of the hill.
Although legal, these quarries were allowed to run despite running the risk of landslides;
10 out of the 11 pockets which witnessed major landslides, and where 91 quarries
operated, were classified as „ecologically sensitive zones‟ and proposed to be banned from
mining and quarrying by the Western Ghats Ecology Expert Panel, also known the
Madhav Gadgil Committee in its report in 2011. After the Gadgil report was criticized as
biased against development, the government constituted another committee, the high-level
working group or the Kasturirangan committee, which recommended a reduced zone of
protection. But even by the Kasturirangan committee report, five out of these 11 landslide
areas should have been banned from mining and quarrying. As per the report, Kerala has a
total 5,924 quarries, an average of six quarries per panchayat, of which 3,332 are in the
ecologically sensitive zones identified by Gadgil. In sum, 56 % of the quarries are on
fragile spots in the Western Ghats, making them prone to landslides.

Based on the type of environmental impacts involved and the ecological sensitivity
of the Western Ghats zone, a graded or layered approach to the regulation and promotion of
construction activities situated in the Western Ghats is recommended by the Western Ghats
Ecology Expert Panel (WGEEP). As per this, the WGEEP has identified the entire Western
Ghats as an Ecologically Sensitive Area (ESA) and allocated three levels of Ecological
Sensitivity to separate regions of it termed as Ecologically Sensitive Zone 1 (i.e. Regions of
highest sensitivity or ESZ1), Ecologically Sensitive Zone 2 (Regions of high sensitivity
orESZ2) and Ecologically Sensitive Zone 3 (Regions of moderate sensitivity or ESZ3).

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 89

Chapter 3
Based on this ESZ1, ESZ2 and ESZ3 levels were assigned within the Western Ghats
frontier to the 30 taluks in Kerala. A total of 25 taluks in 12 districts with 50% or more of
their area is included within the Western Ghats boundary in which, 15 taluks coming under
ESZ1, 2 taluks in ESZ2 and 8 taluks in ESZ3 (Table 3.8). Furthermore, a total of 18 taluks
in 9 districts with less than 50 % of their area is also included within the Western Ghats
boundary in which, 2 taluks coming under ESZ1 and 16 taluks in ESZ2 (Table 3.9). There
can be no doubt that the Western Ghats are a rare biological heritage that needs to be
preserved and nurtured along the path of environmentally and socially sound development.
For this cause, the WGEEP strongly suggested that the entire Western Ghats tract be
considered to be an Ecologically Sensitive Area, with large areas brought under
Ecologically Sensitive Zone 1 and 2. The Peechi-Vazhani and Athirappilly-Vazhachal in
Thrissur district and Periyar are coming under the Ecologically Sensitive Localities (ESL).
As per the suggestion of WGEEP, the Chalakudy River (one of the river that draining into
Vembanad wetland through Periyar River) should be designated as a region rich in fish
diversity to be administrated in Kerala under the „Conservation of biodiversity rich areas of
Udumbanchola taluk‟ pattern. Pronab Sen committee report (2000) categorized the origins
of rivers and wetlands as an Ecologically Sensitive Areas of having intrinsic ecological
service values. The Ecologically Sensitive Zones and Protected areas in Kerala as per
WGEEP are shown in the Figure 3.7.

90 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 3.7 Ecologically Sensitive Zones and Protected Areas of Kerala
(Source: KSBB report, 2011 (Modified after WGEEP report, 2011)

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 91

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Table 3.8 Various taluks in 12 districts of Kerala with 50 % or more of their area is included
within the ESZ1, ESZ2 and ESZ3 of Western Ghats

Sl. Taluks assigned to Taluks assigned to Taluks assigned to
District ESZ1
ESZ2 ESZ3
No.

1 Kasaragod - - Hosdurg

2 Kannur Tellicherry - -

Vythiri, Mananthavady, - -
3 Wayanad

Sultan Bathery

4 Kozhikode - - Mahe

5 Malappuram - - Malappuram

6 Palakkad Mannarkkad, Chittur - Alathur

7 Thrissur Irinjalakuda Thrissur Wadakkanchery

Todupulai,

8 Idukki Udumbanchola, - -

Devikulam, Pirmed

9 Kottayam - Kanjirapally Pala (Lalam)

10 Kollam Punalur - Kottarakkara

11 Pathanamthitta Ranni - Mallappally

12 Thiruvananthapuram Nedumangad - -

(Source: WGEEP report, 2011)

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Table 3.9 Various taluks in 9 districts of Kerala with less than 50 % of area is included within
the ESZ1 and ESZ2 of Western Ghats

Sl. Taluks assigned to Taluks assigned to

District

No. ESZ1 ESZ2

1 Kasaragod - Kasaragod

2 Kannur - Talipparamba

3 Kozhikode Kozhikode Koyilandy, Kozhikode

4 Malappuram - Perinthalmanna, Tirur

5 Palakkad Palakkad Palakkad, Ottappalam

6 Ernakulam Perumbavoor, Aluva,
-

Kothamangalam, Muvattupuzha

7 Kottayam - Changanassery

8 Kollam - Kollam

9 Thiruvananthapuram - Thiruvananthapuram, Chirayinkil

(Source: WGEEP report, 2011)

Geographically, the Western Ghats is the catchment for river systems that drain
almost 40 % of the land area in India. Among the rivers in peninsular India which
originated from the Western Ghats, Godavari, Krishna, Kaveri, Kali Nadi and Periyar
having inter-state importance. The shorter perennial monsoon fed west-flowing rivers like
Sharavati, Netravathi, Periyar, and the Bharathapuzha travel through steeper and more
undulating topography before emptying into the Arabian Sea. Except for a few coastal
streams, one-third of the basin area of most of the river basins is located within the Western
Ghats. Also, the marine and backwater fisheries are maintained by the rich nutrients and
sediments brought down by these flowing rivers. Among the west-flowing rivers in
Western Ghats Periyar, Chalakudy, Keecheri, Puzhakkal, Karuvannur, Muvattupuzha,
Meenachil, Manimala, Pamba and Achankovil rivers drain into Vembanad-Kol wetland
before emptying into the Arabian Sea. Most of these rivers are either dammed or diverted,
some of them for power generation in the upper reaches and irrigation in the lower reaches
at many places. For example, the west-flowing shorter rivers such as Periyar and Pamba

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 93

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have been dammed at several places. By violating all natural laws, west-flowing rivers have
been virtually transformed into east-flowing rivers. The construction of the dam will
entirely modify the ecology of the river system. Hence, dams are without dispute the most
direct modifiers of river flows. They can significantly modify the magnitude (amount) of
water flowing downstream, the timing, frequency and duration of high and low flows and
change the natural rates at which rivers rise and fall during runoff events. Extreme daily
flow fluctuation between peak and off peak times below dams is commonplace in west-
flowing dammed rivers. In addition to cutting off flood plains and influencing aquatic
ecology and riparian systems, drinking water schemes, major and minor irrigation
programmes running in downstream regions have been affected. Similarly, following power
generation, diversion of flows into another river basin causes issues of regular floods in the
recipient basin and drought in the diverted basins. Furthermore, many of the rivers in
Western Ghats are facing the repercussions of indiscriminate sand mining and it is also one
of the serious problems faced by the major rivers associated with the Vembanad wetland
system. The immediate consequences are the reduction of water tables and the degradation
of water quality. Besides, one of the most important ecological issues of considerable
concern is that degradation and pollution of soil and water in the upper reaches of the
Western Ghats gets carried downstream contributing to the degradation of midlands and
coastal areas. In the Western Ghats, the need to curb the usage of chemical pesticides and
fungicides used in plantations is of greater significance than elsewhere, as the use of these
„toxins‟ in the higher hills gets carried downstream polluting the entire wetland system.
Hence, the serious ecological alterations and modifications in most of the west-flowing
rivers which arises due to the various problems in Western Ghats also impacts the ecology
of Vembanad wetland system, as some these rivers empties into Vembanad wetland before
reaches to Arabian Sea.The encroachment of river basins of the Western Ghats for various
developmental activities (i.e. for hydropower or water projects) has raised many concerns.
Therefore, the Vembanad wetland encompassing the ten river system, river basins, riparian
zones and associated dams fall directly or indirectly under the ESZ1, ESZ2 and ESZ3
mostly in the districts of Ernakulam, Kottayam, Idukki, Pathanamthitta and Thrissur.
Therefore, as suggested in the WGEEP report (2011), there is a need for decentralized river
basin planning for west-flowing rivers and maintaining environmental flow in these rivers.
Other major threats associated with riverine ecosystems of the Western Ghats which
include deforestation, development of road infrastructure andplantations, unplanned
tourism and biological invasions. Hence, a policy shift is thus desperately justified in
reducing the environmentally devastating activities takes place in the Western Ghats.

94 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Salient findings...

 The lack of significant storage reservoirs in the upstream of the major rivers,
reduction of depth and shrinkage in carrying capacity of Vembanad wetland, the
structural limitations of Thottappally spillway and the Thanneermukkom
barrage that plays a major role in worsening the 2018 flooding in Kuttanad
region.

 Also, the overall degradation, pollution from various sources and waste
accumulation has led to the loss and reclamation in large areas of the rivers and
wetland habitat significantly modifying the carrying capacity, causing serious
flood condition in the area.

 Vembanad wetland is a notable part of the larger Western Ghats and its
ecological conditions. The floods in 2018 and the subsequent period have
affected the river flow, storage capacity of dams and reservoirs in the
Ecologically Sensitive Zones (ESZ1, ESZ2 and ESZ3) of the wetland. So, a
decentralized river basin planning for west-flowing rivers and maintaining
environmental flow in these rivers is necessary.

 New scientific and development oriented initiatives should be proposed and
implemented in the Western Ghats region considering the ESZ zones and its
conservation strategies. The recent report of the IUCN (2020) has also
highlighted the urgent need for implementation of the Gadgil and
Kasthurirangan recommendations of the WGEEP (2011) report in total. At
present due to various anthropogenic interventions and looming climate change
issues, the Vembanad wetland area under the Western Ghats region has
degraded miserably, also the 2018 floods and subsequent periods have worsened
the situations in the area.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 95

4. IMPACT OF FLOOD AND POST FLOOD ON WATER QUALITY
AND PRODUCTIVITY OF VEMBANAD WETLAND

4.1 Introduction

Water quality is defined in terms of the chemical, physical and biological contents
of water. With the seasons and geographic regions, the water quality of rivers and lakes
varies, even though there is no pollution present. Temperature, rainfall, pH, salinity,
dissolved oxygen (DO), biochemical oxygen demand (BOD), alkalinity and acidity and
heavy metal contaminants are the major physical and chemical parameters affecting the
aquatic environment. These parameters are the limiting factors for the survival of aquatic
organisms (flora and fauna). Poor water quality may be induced by low water flow,
municipal wastewater and industrial discharges (Chitmanat and Traichaiyaporn, 2010).

Estuaries are highly dynamic because they are regulated by freshwater from rivers
and streams mixes with salt water from the marine environment (Silva et al., 2011). They
are an integral part of the hydrological cycle and shows temporal and spatial changes. The
temporal changes ranged from instant hourly variations to long lasting seasonal variations.
These temporal changes are mainly due to changes in meteorology, marine influences such
as tides and saline water influx and riverine influences, such as freshwater flow and
sedimentation. Estuaries are spatially heterogeneous due to gradients in distribution of
hydrographic parameters. The gradient distribution of hydrographic parameters in the
estuary depends upon the morphology, circulation and mixing, sources of dissolved and
suspended constituents and anthropogenic pressure. (Bergamino and Richoux, 2015). The
hydrographic conditions in an ecosystem are also determined by the factors such as
regional precipitation and the temperature resulting in surface cooling and heating. These
spatial, temporal and local changes determine the physical and chemical nature of an
aquatic ecosystem. The influence of the hydrographic parameters over the living
community shows the abiotic and biotic relationship in an environment. The spatio-
temporal variation in the hydrographic parameters in turn affects the community structure
of an ecosystem. The physical parameters like currents and the tidal flow and the
meteorological parameters like seasonal rainfall and evaporation are mainly determining
the hydrographic conditions in an aquatic ecosystem. Changes in these parameters also
effect the distribution of other physical parameters like depth, salinity, temperature and
transparency. Another important factor that determines the dynamic nature of an estuary is
the chemical parameters. In an aquatic ecosystem, the land runoff, sewage, industrial
effluents and sedimentation are the major sources of chemical parameters like pH,
alkalinity and BOD. The distribution of chemical parameters is also affected by the

96 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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physical parameters in the ecosystem. This shows that the dynamic nature of an aquatic
ecosystem is the net effect of its physical and chemical entities.

Estuaries are vulnerable habitats that are experiencing declining water quality and
eutrophication. Anthropogenic activities associated with urbanization, coastal development,
aquaculture and industrial expansion seriously affected the quality of natural habitats in
these sensitive waters. Water may be polluted by several factors including decayed animal
and vegetable matter and living microorganisms such as algae and bacteria, industrial and
commercial solvents, heavy metals, herbicides and pesticides. These factors may constitute
water a bad taste, colour, odour and cause hardness, corrosiveness, staining or frothing. The
quality of water not only retards the availability of potable water, but also decreases
biodiversity and fisheries potential. Eutrophication is another problem facing the estuarine
system of Kerala. The trend in eutrophication is evidenced by the progressive concentration
of nutrients like phosphate and nitrate. The aquatic water bodies satisfy our domestic,
industrial, transport and sporting needs. Alterations in the water quality parameters
seriously affect the dynamic and delicate balance of these pristine aquatic ecosystems. In
the environmental studies, water quality assessment attained an important position because
the physico-chemical characteristics of the aquatic water bodies have gained worldwide
acceptance. The water quality and biodiversity of the estuarine system are degrading day by
day mainly due to increase in human settlements near the shore, industrialization and
urbanization.

Estuaries receive inputs of pollutants, as they are often situated in the vicinities of
highly populated and industrialized areas. Toxic metals are usually present in industrial,
municipal and urban runoff, which can be harmful to humans and biotic life. Increased
urbanization and industrialization are to be blamed for an increased level of trace
metals, especially heavy metals, in our waterways (Seema Singh et al., 2011). Heavy
metals are high priority pollutants because of their relatively high toxic and persistent
nature in the environment. Metal ions can be incorporated into food chains and
concentrated in aquatic organisms to a level that affects their physiological state. In the
present study, water and sediment were analysed for heavy metals viz. copper (Cu), zinc
(Zn), nickel (Ni), cadmium (Cd), lead (Pb) and iron (Fe).

Estuaries provide habitat for species that are valued commercially, ethnically and
recreationally. An increasing world population reaching 7.1 billion people, of which 50 - 60
% are living in the coastal regions and this increased population creates continuous pressure
on coastal zones. Many estuaries are subjected to over exploitation and destruction since
industrial revolution. Human activities directly modify the river ecosystems by drawing
water from water bodies for irrigation, industry and population, dredging, filling and
pollution of rivers and lakes from point and diffusion sources and so on. Nutrients that

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 97

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include many chemicals used as fertilizers in agriculture as well as waste from livestock
and humans are dumped into the estuaries by land runoff. All these activities also alter the
hydrographic parameters such as salinity intrusion, tidal forcing and current speed,
sedimentation, freshwater out flow and residence time of an aquatic ecosystem.

Estuaries provide multiple functions, significant to social, economic and
environmental values. Over the years the availability of domestic and industrial wastes,
urban discharges and agricultural effluents increase the nutrient concentration by many
times to the levels that occur naturally causing several environmental modifications in
estuarine and coastal waters. Many estuaries are subjected to overexploitation and
destruction since industrial revolution. Sustaining good water quality condition is important
not only for ecosystem health, but also for providing service and health to people.

Over the years, the ecobiological status of the Vembanad wetland has under gone
grave modifications mainly due to excessive anthropogenic interventions; particularly
pollution from various sources includes tourism development, reduction and shrinkage of
the water body, habitat change, depletion, extinction of bio resources affecting the
livelihood condition. Several of the changes have been taken place on the hydrology and
environmental quality in the estuary. There has been lot of alterations in the upland areas
of Vembanad, on account of the various projects as well as the increased use of water for
a wide array of activities. There are also in-situ changes in the Vembanad area on account
of various interventions. The implementation of various developmental projects has led to
the loss of productive estuaries along the Kerala coast mainly affecting its life
sustainability. Such demographic impacts have altered the ecology and biotic production of
several coastal estuarine systems on the southwest coast of India (Bijoy Nandan, 2008).
The basic feature of an estuary is the instability that is acting as the main driving force for
healthy estuarine dynamics. The estuarine dynamics depend upon the physical and
chemical characteristics that bring spatial and temporal changes in hydrographic conditions.
In the present study, an assessment has been attempted to evaluate the effects of Kerala
floods 2018 on the water quality parameters of Vembanad wetland system.

4.2 Results

4.2.1 Physical characteristics

The physical characteristics of surface and bottom water like depth, temperature,
transparency and salinity were determined in the study area and its spatial and temporal
variations are explained.

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4.2.1.1 Depth
The average depth in the study area during flood was 3.29 ± 1.78 m. The lowest

depth recorded was 0.7 m at St.7 and highest depth of 8 m recorded at St.5 (Fig. 4.1).
During post flood, the depth ranged between 1 to 8 m (av. 3.26 ± 1.66 m). St.1 recorded
with the highest depth and lowest depth observed in St.7. During the study period, most of
the stations showed a shallow nature, particularly the stations south of TMB. The average
depth decreased during post flood period compared to flood period.

Fig. 4.1 Spatial variation of water depth (m) in Vembanad wetland during flood & post
flood, 2018

4.2.1.2 Transparency
During flood, the maximum transparency of 1 m was observed in St.26, St.28, St.29

and St.30 whereas the minimum value of 0.3 m was observed in St.31 and St.38. Station 15
was also recorded with a lowest value of 0.4 m (Fig. 4.2). The transparency values showed a
uniform distribution in most of the stations. The average transparency of the water column
was 0.65 ± 0.19 m during flood period. The transparency values during post flood were
ranged between 0.2 to 1 m (av. 0.56 ± 0.16 m). The maximum value recorded at St.29 and
minimum value at St.34. Comparatively lower value (0.4 m) was recorded in St 2 and 8.
The average transparency was found to be decreased during post flood period.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 99

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Fig. 4.2 Spatial variation of water transparency (m) in Vembanad wetland during
flood & post flood, 2018

4.2.1.3 Temperature
During flood, the surface water temperature ranged between 28 to 32 °C (av. 30.03

± 0.71 °C). The lowest temperature of 28 °C was observed at St.39 and highest value of 32
°C was recorded at St.38. Stations 10, 11, 12, 13 and 15 were also recorded with
comparatively lower temperature (29 °C) (Fig. 4.3). In bottom water, the values ranged
between 28 to 30°C (av. 29.46 ± 0.72 °C). In most of the stations, temperature showed a
uniform distribution. Comparing the average surface and bottom water temperature, surface
water showed the higher value.

However, during post flood, the surface water temperature ranged between 29 to 31
°C (av. 30.13 ± 0.57 °C). The highest value was recorded at stations 4, 7, 11, 13, 15, 23, 27,
28 and 32 whereas the lowest value recorded at St.1 and St.16. In bottom water, the values
ranged between 28 to 31 °C (av. 29.85 ± 0.63 °C). The highest value recorded at St.5,
St.15, St.25 and St.32 whereas the lowest value recorded at St.17. The average temperature
was maximum during post flood period compared to the flood period. Surface water
showed comparatively higher temperature than bottom water.

100 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 4.3 Spatial variation of surface and bottom water temperature (ºC) in Vembanad wetland
during flood (A) & post flood (B), 2018

4.2.1.4 Salinity
During flood, the salinity of surface water ranged between 0 to 31 ppt (av. 1.28 ±

5.42 ppt) and bottom water values ranged between 0 to 35 ppt (av. 1.62 ± 6.32 ppt). Station
39 recorded with the maximum value for both surface and bottom water (Fig. 4.4).
Whereas, stations 1 to 30 and stations 33 to 38 were recorded with zero. During flood
period most of the stations exhibited a purely limnetic condition. The average salinity value
of bottom water was high as compared to the surface water.

During post flood, the salinity values ranged between 0 to 30 ppt (av. 5.64 ± 9.94
ppt) in surface water and the highest value was observed at St.39. In bottom water, the
values ranged between 0 to 31 ppt (av. 6.18 ± 10.83 ppt). Like surface water, St.39 showed
the highest value. Stations 3 to 30 were recorded with zero during post flood period.
Compared to the flood period, the salinity showed a slightly increasing trend during post
flood. The bottom water showed higher salinity compared to surface water.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 101

Chapter 4

Fig. 4.4 Spatial variation of surface and bottom water salinity (ppt) in Vembanad wetland
during flood (A) & post flood (B), 2018

4.2.2 Chemical characteristics
The chemical characteristics of surface and bottom water in the study area were

determined. pH, alkalinity, dissolved oxygen, biological oxygen demand, hydrogen sulphide
and inorganic nutrients were determined.
4.2.2.1 pH

During flood, the average pH of surface water was found to be 5.96 ± 0.79 and it
ranged between 4.8 to 8.67. The maximum value was observed at St.34 and minimum value
observed at St.12. In bottom water, the values varied from 4.35 to 8.05 (av. 5.99 ± 0.79). Like
surface water, St.12 showed the lowest value and St.32 showed the highest value. pH values
showed wide variation in most of the stations, particularly stations south of TMB (Fig. 4.5).
During flood, the average pH of bottom water was high when compared to the surface
water.

During post flood, the surface water pH varied from a lower value of 4.92 observed
at station 13 and a higher value of 7.68 observed at station 31 with an average value of 6.17 ±
0.84. In bottom water, the values ranged between 6.02 to 7.63 with an average value of 6.69
± 0.52. Station 32 recorded with the highest value and St.22 recorded with the lowest value.
The pH value of most of the stations increased during post flood period compared to that of
flood period. Bottom water recorded with the highest pH compared to surface water.

102 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 4.5 Spatial variation of surface and bottom water pH in Vembanad wetland during
flood (A) & post flood (B), 2018

4.2.2.2 Alkalinity
During flood, the alkalinity of surface water ranged between 15 to 55 mg L-1

(av. 22.44 ± 6.97 mg L-1). The maximum value was recorded at St.39 and minimum value
at St.3, St.4, St.21 and St.37 (Fig. 4.6). The bottom water alkalinity values ranged between
15 to 100 mg L-1 (av. 23.85 ± 13.59 mg L-1). Like surface water St.39 recorded with the
highest value, whereas the lowest value observed at St.1, St.20, St.21, St.34, St.36 and
St.37. Alkalinity values showed a uniform distribution in most of the stations. During flood,
bottom water showed comparatively higher value than the surface water.

During post flood, the surface water alkalinity varied from a lower value of 20 mg
L-1 observed at stations 16, 25 and 28 and a higher value of 95 mg L-1 observed at St.1 with
an average value of 31.15 ± 13.69 mg L-1. In bottom water, the alkalinity values ranged
between 20 to 75 mg L-1(av. 29.49 ± 9.72 mg L-1). The maximum value was recorded at
St.1 and minimum value observed at St.6, 10, 17 and 21. Alkalinity values were higher
during post flood period compared to that of flood period. Surface water showed higher
alkalinity compared to bottom water during post flood.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 103

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Fig. 4.6 Spatial variation of surface and bottom water alkalinity (mg L-1) in Vembanad
wetland during flood (A) & post flood (B), 2018

4.2.2.3 Dissolved oxygen
During flood, the DO values in surface water ranged between 4.7 to 10.2 mg L-1 (av.

7.26 ± 1.22 mg L-1). The lowest value recorded at St.16 and St.39 whereas the highest value
observed at St.8. In bottom water, the DO varied from a lower value of 3.15 mg L-1
observed at station 8 and a higher value of 10.23 mg L-1 recorded at station 2 with an
average value of 7.33 ± 1.38 mg L-1(Fig. 4.7). The DO showed higher values in most of the
stations during flood period. Bottom water showed comparatively higher DO value than the
surface water.

During post flood, the surface water DO varied from a lower value of 3.1 mg L-1
recorded at St.31 and a higher value of 10.6 mg L-1 observed at St.37 with an average value
of 6.56 ± 1.80 mg L-1. The bottom water DO values ranged between 3.07 to 10.47 mg L-
1(av. 7.11 ± 1.52 mg L-1). Similar to the surface water the maximum value observed at
St.37 and minimum value observed at St.31. The average DO concentration observed
during post flood was lower as compared to the flood period. The bottom water showed
maximum value during post flood.

104 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 4.7 Spatial variation of surface and bottom water DO (mg L-1) in Vembanad wetland
during flood (A) &post floo d (B), 2018

4.2.2.4 Biological oxygen demand (BOD)
During flood, the BOD in surface water varied from a lower value of 0.79 mg L-1

(St.17) to a higher value of 6.3 mg L-1 (St.8 and St.15) with an average value of 3.71 ± 1.42
mg L-1. In bottom water, the values ranged between 0.79 to 6.3 mg L-1 (av. 3.69 ± 1.33 mg
L-1). Station 15 showed the maximum value whereas St.8 and St.14 showed the minimum
value (Fig. 4.8). During flood, most of the stations were recorded with comparatively
higher BOD values. The surface water showed comparatively higher BOD value than the
bottom water.

During post flood, the surface water BOD values ranged between 0.79 to 5.51 mg L-1
(av. 2.79 ± 1.02 mg L-1). The highest value was observed at St.14 and lowest value at St.10.
In bottom water, the BOD varied from a minimum value of 1.5 mg L-1 (St.33) to a
maximum value of 5.51 mg L-1 (St.28) with an average value of 3.12 ± 1.01 mg L-1. BOD
showed maximum value during flood period compared to the post flood period. Bottom
water showed higher BOD value during post flood.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 105

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Fig. 4.8 Spatial variation of surface and bottom water BOD (mg L-1) in Vembanad wetland
during flood (A) & post flood (B), 2018

4.2.2.5 Hydrogen sulphide (H2S)
During flood, the average hydrogen sulphide concentration in surface water was

found to be 1.05 ± 1.49 µmol L-1 and it ranged between 0.05 to 7.93 µmol L-1. The
maximum value was observed at St.12 and minimum value at St.2, St.3, St.9, St.22 and
St.39 (Fig. 4.9). In bottom water, the values ranged between 0.05 to 3.71 µmol L-1 (av. 0.78
± 0.91 µmol L-1) while St. 37 recorded the highest value. Like surface water St.12 was also
showed comparatively higher concentration. During flood period, surface water showed
higher value.

During post flood, the surface water H2S varied from a lower value of 0.05 µmol L-1
observed at St.28 and a higher value of 9.62 µmol L-1 at St.13 (av. 2.88 ± 2.60 µmol L-1). In
bottom water, the values ranged between 0.05 to 8.49 µmol L-1 (av. 2.39 ± 1.70 µmol L-1).
Station 9 recorded with the maximum value and the minimum value observed at St.15.
During the study period, the southernmost stations (stations south of TMB) recorded the
higher H2S concentration. The average H2S concentration was higher during post flood
compared to the flood. Like flood period, surface water showed higher H2S concentration
than bottom water.

106 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 4.9 Spatial variation of surface and bottom water hydrogen sulphide (µmol L-1) in
Vembanad wetland during flood (A) & post flood (B), 2018

4.2.3 Inorganic Nutrients
4.2.3.1 Phosphate-Phosphorus

During flood, the phosphate-phosphorus concentration in surface water ranged
between 3.28 to 27.55 µmol L-1 (av. 10.44 ± 6.36 µmol L-1). Station 39 showed the highest
value and St.6 showed the lowest value(Fig. 4.10).The average bottom water concentration
was found to be 5.58 ± 3.17µmol L-1 and it ranged between 3.28 to 15.35 µmol L-1. The
maximum value was observed at St.37 and minimum value at St.3, St.27 and St.29. Surface
water showed higher phosphate concentration than bottom water.

During post flood, the surface water PO4-P concentration varied from a lower value
of 0.89 µmol L-1 recorded at St.37 and a higher value of 23.06 µmol L-1 observed at St.1
with an average value of 7.41 ± 4.94µmol L-1. In bottom water, the values ranged between
2.96 to 20.94 µmol L-1 (av. 7.67 ± 4.22 µmol L-1). The highest value was observed at St.31
and lowest value at St.37. Like surface water St.1 showed comparatively higher value
(14.07 µmol L-1). The average PO4-P displayed higher concentration during flood than post
flood period. The bottom water showed higher concentration than surface water.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 107

Chapter 4

Fig. 4.10 Spatial variation of surface and bottom water phosphate (µmol L-1) in Vembanad
wetland during flood (A) & post flood (B), 2018

4.2.3.2 Silicate-Silicon
During flood, the SiO4-Si concentration in surface water ranged between 20.75 to

158.13 µmol L-1 (av. 59.83 ± 34.96 µmol L-1). Highest value was recorded at St.15 and
lowest value at St.31. In bottom water, the values ranged between 13.27 to 124.04 µmol L-1
(av. 59.15 ± 29.55µmol L-1). Station 15 showed the maximum value whereas the minimum
value observed at St.2 (Fig. 4.11). Most of the stations were recorded with higher SiO4-Si
values during flood period. The surface water showed slightly higher silicate concentration
than bottom water.

During post flood, the average SiO4-Si in surface water was found to be 28.76 ±
15.32 µmol L-1 and it ranged between 0.72 to 63.45 µmol L-1. The highest value observed
at St.17 and lowest value at St.37. In bottom water, the SiO4-Si varied from a lower value
of 2.98 µmol L-1(St.37) to a higher value of 35.05 µmol L-1 (St.19) with an average value of
22.12 ± 7.82 µmol L-1. In the present study, SiO4-Si showed maximum concentration
during flood compared to the post flood. The surface water showed higher silicate
concentration than bottom water.

108 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 4.11 Spatial variation of surface and bottom water silicate (µmol L-1) in Vembanad
wetland during flood (A) & post flood (B), 2018

4.2.3.3 Ammonia-Nitrogen
During flood, the concentration of NH4-N in surface water ranged between 0.03 to

18.11 µmol L-1 and the estimated average concentration was found to be 3.12 ± 3.82 µmol
L-1. The maximum concentration was observed at St.37 and minimum concentration at
St.30 (Fig. 4.12). The bottom water NH4-N varied from a lower value of 0.25 µmol L-1
(St.6) to a higher value of 12.7 µmol L-1(St.37) with an average value of 3.40 ± 3.39 µmol
L-1. The average bottom water ammonia concentration was higher compared to the surface
water.

During post flood, the average NH4-N concentration in surface water was found to
be 8.80 ± 6.80 µmol L-1 and it ranged between 0.83 to 33.86 µmol L-1. Station 37 recorded
the highest value and St.25 showed the lowest value. The stations south of TMB were also
recorded with higher values during post flood period, among this St.10 - Nehru trophy
finishing point (22.13 µmol L-1) and St.12 - Punnamada (20.68 µmol L-1) showed the
higher concentration. In bottom water, the values ranged between 1.9 to 28.91 µmol L-1
(av. 9.44 ± 5.88 µmol L-1). Like surface water, the maximum value was recorded at St.37
whereas the minimum value at St.17. The NH4-N showed an increasing trend from flood to
post flood period. Bottom water showed higher concentration compared to surface water.

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 109

Chapter 4

Fig. 4.12 Spatial variation of surface and bottom water ammonia (µmol L-1) in Vembanad
wetland during flood (A) & post flood (B), 2018

4.2.3.4 Nitrite-Nitrogen
During flood period, the NO2-N concentration in surface varied from a lower value

of 0.65 µmol L-1 (St.16) to a higher value of 1.58 µmol L-1 (St.31) with an average value of
0.92 ± 0.24 µmol L-1 (Fig. 4.13). The average concentration of NO2-N in bottom water was
found to be 0.78 ± 0.11 µmol L-1 and it varied from a lower value of 0.65 µmol L-1 (St.2,
St.5, St.18 and St.30) to a higher value of 1.06 µmol L-1(St.33). The surface water showed
higher nitrite concentration than bottom water.

During post flood, the nitrite concentration in surface water ranged between 0.15 to
3.36 µmol L-1 (av. 1.18 ± 0.85 µmol L-1). The maximum value was observed at St.1 and
minimum at St.38. The concentration of NO2-N in bottom water ranged between 0.11 to 2.9
µmol L-1 and the estimated average concentration was found to be 0.84 ± 0.57 µmol L-1.
Station 2 showed the highest value whereas St.38 showed the lowest value. The average
surface water nitrite concentration was higher compared to the bottom water.

110 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Fig. 4.13 Spatial variation of surface and bottom water nitrite (µmol L-1) in Vembanad wetland
during flood (A) & post flood (B), 2018

4.2.3.5 Nitrate-Nitrogen
During flood, the average NO3-N concentration in surface water was found to be

2.91 ± 2.18 µmol L-1 and it ranged between 1.29 to 9.22 µmol L-1. The maximum value
observed at St.37 and minimum at St.6 and St.29. (Fig. 4.14). In bottom water, the values
ranged between 1.29 to 14.98 µmol L-1 (av. 2.53 ± 2.47 µmol L-1). The highest value
recorded at St.39 and lowest at St.18 and St.30. The surface water showed higher
concentration than bottom water.

During post flood period, the nitrate concentration in surface water varied from a
lower value of 1.33 µmol L-1 (St.5 and St.17) to a higher value of 33.14 µmol L-1 (St.39)
with an average value of 3.93 ± 5.46 µmol L-1. Like flood period, St.1 displayed
comparatively higher value (9.22 µmol L-1). The bottom water values ranged between 1.29
to 21.18 µmol L-1 (av. 3.67 ± 3.86 µmol L-1). Station 39 showed the highest value and
lowest value observed at St.6 and St.14. The NO3-N showed maximum concentration
during post flood as compared to the flood. Like in flood period, the NO3-N showed higher
concentration in surface water than bottom water.

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Chapter 4

Fig. 4.14 Spatial variation of surface and bottom water nitrate (µmol L-1) in Vembanad wetland
during flood (A) & post flood (B), 2018

4.2.4 Primary productivity
During flood, the surface water gross primary productivity (GPP) ranged between 0.74

to 3.69 gC m-3 day-1 (av. 1.70 ± 0.81 gC m-3 day-1). Station 8 showed the highest value of 3.69
gC m-3 day-1 whereas the lowest value of 0.74 gC m-3 day-1 was recorded at St.5, St.6, St.13,
St.14, St.18, St.20, St.21, St.29, St.31, St.33 and St.37 (Fig. 4.15). In bottom water, the GPP
values ranged between 0.74 to 4.43 gC m-3 day-1 (av. 1.61 ± 0.88 gC m-3 day-1). The highest
value was observed at St.2 whereas the lowest value of 0.74 gC m-3 day-1 recorded in most of
the stations. The net primary productivity (NPP) in surface water varied from 0.74 to 2.21
gC m-3 day-1 (av. 1.10 ± 0.47 gC m-3 day-1). Stations10, 14 and 16 were recorded the highest
value and most of the stations were recorded the lowest value of 0.74gC m-3 day-1 (Fig.
4.16). The bottom water NPP values ranged between 0.74 to 5.17 gC m-3 day-1 (av. 1.08 ±
0.77 gC m-3 day-1) and the maximum value was observed at St.8.

During post flood, the surface water GPP values ranged between 1.48 to 5.54gC m-3
day-1 (av. 4.10 ± 1.00 gC m-3 day-1). Station 35 was recorded the highest value and St.38
recorded the lowest value. In bottom water, the GPP values varied from 0.74 to 5.9gC m-3 day-
1 (av. 3.24 ± 1.26 gC m-3 day-1). The maximum value was observed at St.39 whereas the
minimum at St.17, St.21 and St.22. The NPP values in surface water varied from 0.74 to
5.17gC m-3 day-1 (av. 2.02 ± 1.25 gC m-3 day-1) and the highest value was observed at St.33.
The average NPP in bottom water was found to be (av. 1.56 ± 1.01 gC m-3 day-1) and it
ranged between 0.07 to 4.5gC m-3 day-1. The highest value was observed at St.39 and the
lowest at St.32. During the study period, GPP showed the highest value compared to NPP

112 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

Chapter 4
and the maximum productivity observed during post flood period. In the present study,
surface water recorded with the maximum GPP and NPP compared to bottom water.

Fig. 4.15 Spatial variation of surface and bottom water GPP (gC m-3 day-1) in Vembanad wetland
during flood (A) & post flood (B), 2018

Fig. 4.16 Spatial variation of surface and bottom water NPP (gC m-3 day-1) in Vembanad wetland
during flood (A) &post flood (B), 2018

4.2.5 Distribution of heavy metals in water
The distribution of copper in water samples during flood ranged from BDL to

4.375 µg L-1 (Fig. 4.17). The highest value of copper was noted in station 20 and the lowest
value wasnoted in all the stations except 6, 25, 26 and 30. During post flood, the Cu
concentration ranged from 0.25 to 8.06µg L-1(Fig. 4.18). The highest value was found in

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 113

Chapter 4

station 39 and the lowest value in station 15. In the study Cu showed a positive correlation
with water bound Fe (r = 0.173) [p<0.05], transparency (r = 304), BOD (r = 0.224), silicate
(r = 280) and sediment bound Ni (r = 0.393)[p<0.01].

In the present study, the nickel and cadmium concentrations in all the stations were
BDL during flood (Fig. 4.17). In the case of post flood, the concentration of Ni ranged from
BDL to 9.06 µg L-1(Fig. 4.18). The highest concentration was found in station 19. The Ni
was found to be positively correlated with transparency (r = 0.173), silicate (r= 0.176)
[p<0.05] and sediment bound Cu (r = 0.393) [p<0.01]. In case of Cd, the concentration
ranged from BDL to 9.38 µg L-1. The highest concentration was observed in station 32. Cd
showed a positive correlation with salinity (r = 0.406), pH (r = 0.335), GPP (r = 0.294),
NPP (r = 0.435), ammonia (r = 0.423), nitrate (r = 0.222), water bound Cd (r = 0.562), Zn
(0.315), Fe (r = 0.225), sediment bound metals like Pb (r = 528) and Zn (r = 595) [p<0.01].

The concentration of lead during flood ranged from BDL to 9.38µg L-1. The
maximum concentration was found in station 2 and 24 (Fig. 4.17). During post flood, the
lead concentration ranged from BDL to 46.26 µg L-1(Fig. 4.18). The maximum
concentration was noted in station 19. In the study, the Pb was found positively correlated
with DO (r = 0.197), salinity (r = 0.166), pH (r = 0.187), water bound Fe (r = 0.204)
[p<0.05], NPP (r = 0.217), ammonia (r = 245), water bound metals like Cd (r =0.576), Zn (r
= 0.407) and Fe (r = 0.204) [p<0.01].

The concentration of zinc was maximum (145.63 µg L-1) in station 26 and
minimum (1.25 µg L-1) in station 28 during flood (Fig. 4.17). During post flood, the Zn
concentration ranged from 4.16 to 166.25µg L-1 (Fig. 4.18). The high concentration was in
station 39 and low in station 15. Zn showed a positive correlation with sediment bound DO
(r = 0.185), nitrate (r = 0.166), Zn (r = 0.179) [p<0.05], salinity (r = 0.244), pH (r = 0.334),
NPP (r = 0.296), ammonia (r = 0.417), water bound metals like Cd (r = 0.521) and Fe (r =
0.235), sediment bound metals like Cd (r = 0.595) and lead (r = 0.542) [p<0.01].

In the study during flood, maximum iron concentration of 918.75µg L-1 was found in
station 2 and a minimum concentration of 4.75µg L-1 in station 28 (Fig. 4.17). During post
flood, the concentration of Fe ranged from 103.44 to 2202.5 µg L-1(Fig. 4.18). The high
concentration was noted in station 34 and low concentration in station 2. In the study, Fe
showed a positive correlation with transparency (r = 0.229) and water bound Fe (r = 0.268)
[p<0.01].

114 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

Chapter 4

Fig. 4.17 Spatial variation of heavy metals in water samples (µg L-1) in Vembanad wetland during
flood, 2018

Fig. 4.18 Spatial variation of heavy metals in water samples (µg L-1) in Vembanad wetland during
post flood, 2018

4.2.6 Data analysis
4.2.6.1 Principal Component Analysis (PCA)

A multivariate correlation analysis (PCA) was carried out to determine the
distribution and influence of various physico-chemical parameters in Vembanad wetland
(Fig. 4.19). Principal component analysis (PCA) extracted two principal components (PCs)
from the variance present in the data. The PCA was carried out with factors having
eigenvectors higher than one. Factor loadings (correlations between the variables and the

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 115

Chapter 4

extracted factors) for the two retain Eigen values are given in Table 4.1. The first PC
accounted for 21.8 % variability with an Eigen value of 6.1 and second PC accounted for
10.6 % variability with Eigen value of 2.98, whereas the remaining PCs each explained ˂
10 % of the variation. The variables with highest loadings on PC1 were transparency, BOD
and silicate. The second PC had higher loadings for alkalinity, nitrite, sulphide, water bound
Cu, Ni and Pb.

Table 4.1 Results of PCA analysis of physico-chemical parameters in Vembanad wetland
during flood and post flood period, 2018

Eigen values PC1 PC2 PC3 PC4 PC5
% Variation 6.1 2.98 2.22 2.21 1.56
Cum % Variation 21.8 10.6 7.9 7.9 5.6
Variable 21.8 32.4 40.3 48.2 53.8
Depth
Temperature -0.0307.106 --00..006790 -0.2504.106 -0.069 -0.254
Transparency -0.350 0.074
Salinity 0.002 -0.120 -0.091 -0.104 0.065
pH -0.009 0.001
Alkalinity 0.240 -0.047 -0.159 -0.163 0.139
DO 0.073 0.018
BOD -0.333 -0.062 0.234 -0.497 0.044
Sulphide -0.410 -0.009
Ammonia -0.299 -0.079 0.140 0.161 -0.018
Nitrate 0.019 0.178
Nitrite -0.247 0.267 0.101 -0.079 0.147
Phosphate 0.094 0.165
Silicate 0.024 -0.210 -0.096 0.025 0.056
GPP 0.161 -0.024
0.126 -0.185 -0.149 -0.058 -0.131

-0.091 0.133 -0.463

-0.262 0.005 -0.308

-0.315 -0.039 0.205

0.092 0.302 -0.193

-0.112 0.095 0.277

0.321 0.069 0.143

-0.217 0.075 -0.338

116 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

Chapter 4

NPP -0.205 -0.046 -0.046 0.264 -0.124

Cu (Water) -0.202 0.110 -0.393 0.061 -0.188

Ni (Water) -0.146 0.136 0.080 -0.074 -0.453

Cd (Water) -0.160 -0.291 -0.125 0.055 -0.277

Pb (Water) 0.006 0.160 0.133 -0.207 -0.478

Zn (Water) -0.229 -0.099 0.063 0.050 -0.062

Fe (Water) 0.095 -0.249 -0.035 0.086 -0.416

Cu (Sediment) 0.238 -0.212 -0.072 0.201 -0.054

Ni (Sediment) 0.127 -0.090 0.098 0.318 -0.074

Cd (Sediment) -0.198 -0.332 -0.044 0.139 0.081

Pb (Sediment) -0.007 -0.352 -0.120 0.134 0.093

Zn (Sediment) -0.079 -0.396 0.084 0.177 0.129

Fe (Sediment) 0.083 -0.173 0.007 0.010 -0.156

(PCA code- DO-Dissolved oxygen; BOD-Biological oxygen demand; GPP-Gross primary productivity;
NPP-Net primary productivity; Cu- Copper; Ni-Nickel; Cd-Cadmium; Pb-Lead; Zn-Zinc; Fe-Iron)

Study on the impact of heavy floods on environmental characteristics of Vembanad backwater 117

Chapter 4

Fig. 4.19 PCA ordination of physico-chemical parameters in Vembanad wetland during
flood and post flood period, 2018

4.2.6.2 Multiple regression analysis
Multiple regression analysis was carried out to determine the edaphic variables

(independent variables) like rainfall and river discharge influencing the physico-chemical
parameters in the study area during flood and post flood period.
4.2.6.2.1 Rainfall

Multiple regression analysis revealed that rainfall had significant influences on the
factors like alkalinity and ammonia during the flood (p<0.05) (Table 4.2). But in case of
post flood, it influenced transparency (p<0.05), temperature, salinity, NPP and water bound
Pb and Fe (p<0.01).

118 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater

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Table 4.2 Multiple regression analysis of rainfall influencing the physico-chemical and in the
Vembanad wetland during flood & post flood, 2018 (R2 = 0.660)

Variables β Flood p value Post flood
-.095 t .437 β t p value
Temperature .230 .109 -2.532 -3.335 .002**
Transparency -.101 -.784 .500 13.784 2.636 .011*
Salinity .014 1.633 .963 -.506 -2.959 .005**
DO -.331 -.679 .060 -.956 -1.619 .112
BOD -.339 .046 .028* 1.271 1.479 .146
Alkalinity .049 -1.922 .878 -.091 -.874 .387
pH .144 -2.258 .484 2.198 1.494 .142
GPP .049 .155 .747 1.064 2.124 .039
NPP -.518 .705 .001* -1.794 -2.715 .009**
NH3 .049 .325 .819 .069 .582 .563
Phosphate .065 -3.473 .666 .310 1.582 .121
Nitrite -.172 .231 .264 .485 .466 .644
Sulphide -.113 .434 .453 .027 .076 .940
Nitrate .130 -1.130 .651 -.496 -1.959 .056
Silicate .110 -.756 .500 -.082 -.968 .338
Cu in water -.026 .455 .872 -.471 -1.086 .283
Pb in water .064 .680 .636 -2.085 3.011 .004**
Zn in water -.186 -.162 .404 1.911 -1.576 .122
Fe in water .438 .476 .008* .448 -3.184 .003**
Cu in sediment .147 -.841 .423 -.052 -1.679 .100
Ni in sediment -.205 2.784 .306 -.008 .827 .412
Cd in sediment -.399 .807 .101 -.047 -.588 .559
Pb in sediment -1.034 .021 .158 .875
-1.671

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Chapter 4

Zn in sediment .634 2.792 .007* -.246 -.657 .515

Fe in sediment .137 .724 .472 .003 .170 .866

Zooplankton -.326 -2.045 .046* -.003 -.427 .671

Phytoplankton -.038 -.214 .832 8.526 2.063 .045*

Benthos -.565 -2.330 .024* -1.73 .104 .917

∗=statistically significant at 0.05 level of significance

∗∗=statistically significant at 0.01 level of significance

4.2.6.2.2 River discharge

Multiple regression analysis revealed that river discharge showed significant
influences on the factors like alkalinity, nitrite, nitrate and water bound Zn and Fe during
flood (p<0.01) (Table 4.3). But in the case of post flood, it influenced the sulphide
(p<0.01).

Table 4.3 Multiple regression analysis of river discharge influencing the physico-chemical
parameters in the Vembanad wetland during flood & post flood, 2018(R2 =0.660)

Variables β Flood p value Post flood
99.11 t β t p value
Temperature -276.49
Transparency 30.83 .488 .63 145.49 .310 .758
Salinity -91.70 -.330
DO -56.48 .586 .74 2425.365 .750 .457
BOD -55.80 -.653
Alkalinity 162.86 -.481 .56 2.014 .019 .985
pH -.173 -1.95
GPP -281.79 .613 .52 257.506 .706 .484
NPP -30.77 .000
NH3 4.05 -1.11 .63 -100.139 -.188 .851
Phosphate -1565.42 -.54
Nitrite .163 .05* 84.190 1.304 .199
-2.16
.54 -532.734 -.585 .561

.99 -283.742 -.916 .365

.27 -327.871 -.802 .427

.59 -15.178 -.208 .836

.87 -38.575 -.319 .751

.04* -820.136 -1.274 .209

120 Study on the impact of heavy floods on environmental characteristics of Vembanad backwater